1. Field of the Invention
The present invention relates to fungicidal compositions based on (N-phenylacetyl-N-2,6-xylyl)methyl alaninate.
2. Description of the Background
More specifically, the present invention relates to fungicidal compositions comprising the compound corresponding to (N-phenylacetyl-N-2,6xylyl)methyl alaninate, in which more than 50% of said compound consists of the laevorotatory enantiomorph, and one or more known fungicides and their use in the agricultural field for controlling fungine disease which seriously damage agricultural crops.
Many of the compounds which are used for controlling phytopathogen fungi in agrarian practice have at least one asymmetrical centre. In particular, when these compounds have only one asymmetrical centre, or chiral centre, the two enantiomorphs can have a different fungicidal activity.
More specifically, when only one of the enantiomorphs has a significant biological activity, it is possible for the same fungicidal effect to be obtained using the more efficient enantiomorph at a dosage which is half that of the raceme. Obviously when both enantiomorphs have a comparable biological activity, the reductions in the expected applicative dosages should in any case be less than 50% with respect to those of the raceme.
Most fungicidal compounds containing at least one chiral centre are normally sold as a racemic mixture as it is difficult to compensate the higher cost relating to the production of the enantiomorph in its pure form, with additional commercial advantages.
The necessity of improving the environmental impact has recently led to a re-evaluation of the use of single enantiomorphs to obtain at least partial reductions in the dosages used, thus diminishing the quantity of xenogene substances dispersed in the environment and improving the environmental impact of phytoiatric treatment.
In addition, it is particularly advantageous if products with a fungicidal activity can be easily degraded by the vegetable host at the end of the period for which the fungicidal activity is required, so as to guarantee the minimum quantity of residual active principle.
The reduction in residues of main principle in vegetables when they are picked, is in fact linked to potential risk for possible consumers of the agricultural product: the smaller the quantity of residual active principle, the lower the risk will be for the consumer.
The compound (N-phenylacetyl-N-2,6-xylyl)-DL-methyl alaninate, also known under the trade-name of Benalaxyl, is particularly efficient in the control of diseases caused by Oomycetes. Oomycetes are responsible for many diseases of economically important crops such as, for example, grapes, potatoes, tomatoes and tabacco. This fungicide is described in U.S. Pat. Nos. 4,291,049 and 4,425,357.
Benalaxyl has one asymmetrical centre and consists of an equimolecular mixture of the two enantiomorphs. When prepared according to the methods described in the two U.S. patents cited above, Benalaxyl is obtained as a racemic mixture in which the enantiomorphs are present in equimolecular quantities.
"Pesticide Science" (1985), Vol. 16, pages 277-286, on the other hand also describes the preparation of the laevorotatory enantiomorph, corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate and shows its greater activity with respect to both the dextrorotatory enantiomorph corresponding to (N-phenylacetyl-N-2,6-xylyl)-L-methyl alaninate and the raceme, correspondingto (N-phenylacetyl-N-2,6-xylyl)-DL-methyl alaninate. This greater activity has been observed by both experimenting the laevorotatory enantiomorph in tests in vitro, and by applying it on infected plants or on plants infested after its application, and also by applying it to the earth or plant seed, to control the pathogenes present in the soil.
Racemic Benalaxyl is degraded with a different rate depending on the type of vegetable and portion to which it is applied.
For example, the degradation rate of racemic Benalaxyl in hops makes the use of this product unsuitable as an antifungal agent, to the detriment of its distinct fungicidal efficiency: in fact, after two weeks following treatment, the residual levels of active principle are still high.
Together with its good preventive activity, Benalaxyl also has considerable curative efficiency: it is in fact capable of blocking infections already in development consequently allowing intervention after infection. In agrarian practice it is customary to intervene within 72 hours of a possible infection such as, for example, rainfall or abundant night-dew.
Literature of the known art specifies that the MIC (minimum concentration of active principle necessary for inhibiting the development of disease) of the laevorotatory enantiomorph of Benalaxyl, when applied on the leaves of vines cultivated in a vase and infected 24 hours earlier with spores of Plasmopara viticola, is 5 mg/l, the MIC of the dextrorotatory enantiomorph is 100 mg/l and the MIC of the raceme is 10 mg/l.